1. Field of the Invention
The present invention relates to a motor driving circuit which drives a single phase brushless DC motor.
2. Description of the Related Art
A motor driving method has been suggested which amplifies an output voltage signal (hereinafter, referred to as Hall signal) of a Hall device provided to a single phase brushless DC motor and applies the same to a motor coil (for example, refer to JP 2004-140897A).
When generating a driving signal for driving a motor coil based on a Hall signal having a sinusoidal wave form, a waveform of the driving signal is distorted due to distortion of a magnetic flux which the Hall device detects in the vicinity of zero cross, so that rotation becomes unstable or vibration or noise is generated.
In order to solve the this problem, when performing a soft switching operation by a PWM signal for generating the driving signal based on a voltage amplitude of the Hall signal at the time of startup of the motor, a variety of problems may occur due to the voltage amplitude of the Hall signal when performing the soft switching operation. That is, when the voltage amplitude of the Hall signal is large, a back electromotive voltage or noise is generated due to shortness of soft switching time, and when the voltage amplitude is small, the soft switching time is long, so that rotating speed is decreased.